Quod erat demonstrandum

Latin for what was to be shown, or what was to be proven. Apparently coined by Euklides, a.k.a, Euclid of Alexandria (lived around 330-275 BCE) who allegedly used it (or, rather, the abbreviationQ.E.D.) at the end of most of his proofs.

I said allegedly because, as far as I know, Euklides wrote his books in Greek and they were only later translated to Latin by someone else. It is, however, not unusual for many phrases that were originally uttered (or written) in Greek to have been preserved to us in Latin.